1. Field of the Art
This invention relates generally to an ultrasound image processing system for generating and displaying three-dimensional ultrasound images on the basis of a series of sequentially captured two-dimensional tomographic ultrasound images, and more particularly to an ultrasound image processing system having a marker means for marking and distinguishing two-dimensional ultrasound images, which are fundamentally necessary for three-dimensional image processing, from other 2D ultrasound images captured.
2. Prior Art
In ultrasound examination, ultrasound signals are transmitted into patient body through an ultrasound transducer, while thereby receiving return echo signals from body tissues at different depths in the directions of signal transmission. The return echo signals are processed into video signals to display ultrasound images on a monitor screen. A two-dimensional tomographic ultrasound image (hereinafter referred to as "2D ultrasound image" for brevity) is obtained by an ultrasound scan over a predetermined range, that is to say, by a B-mode ultrasound scan of a plane section. A large number of 2D ultrasound images of different plane sections, which are captured successively by shifting the scan position in a certain pitch in a predetermined direction, can be converted into a three-dimensional ultrasound image (hereinafter referred to as "3D ultrasound image" for brevity) through image processing operations. Needless to say, as compared with 2D ultrasound images, 3D ultrasound images displayed on a viewing screen are more helpful in clearly grasping the internal tissue structures of scanned regions, and contribute to enhance the accuracy of ultrasound examinations.
A series of 2D ultrasound images, which are each expressed on a 2D X-Y coordinate system, come to have expressions of spatial expanse when lined up along Z-axis of a 3D X-Y-Z coordinate system. In ultrasound images, echoes from internal tissue structures are converted into different light intensities and expressed as variations in luminance on a viewing screen. Therefore, internal tissue structures can be displayed as a 3D image by implementing picture data between adjacent 2D ultrasound images, through linear interpolation based on luminance levels of picture signals in preceding and succeeding 2D ultrasound images. Further, internal tissue structures can be displayed as 3D ultrasound images by dissolving a 3D space on a 3D coordinate system into voxels which contain the luminance information in the entire 3D space scanned. Images of an organ or internal tissue structures of particular interest can be extracted and displayed by image processing based on 3D picture signals.
Any way, in either type of the above-mentioned 3D picture images, a delimited 3D space is set up in a particular intracorporeal region of interest by way of the scan range of 2D ultrasound picture images and the direction of alignment of the ultrasound picture images, for the purpose of displaying internal tissue structures or an internal organ in that space in a 3D perspective view. In order to acquire picture data for 3D images of this sort, it is necessary to produce all the luminance information throughout that 3D space, namely, to create correlated picture data for and between N-number of 2D ultrasound images captured throughout that space. Therefore, 3D ultrasound image processing normally involves a vast amount of picture data and involves extremely complicate data processing operations which take time even by a large-scale data processor.
In this regard, in an attempt to make it possible to observe 3D ultrasound images of internal tissues quickly on a monitor screen, the inventor developed an inexpensive 3D ultrasound image processing system which can produce 3D ultrasound images through simplified signal processing operations, as disclosed in his U.S. Pat. No. 5,682,295 (hereinafter referred to as "patent" for brevity).
More specifically, according to the 3D ultrasound image processing system of the above-mentioned patent by the inventor, in order to permit three-dimensional grasping of a subject under observation, a 3D ultrasound image is built on X-, Y- and Z-axes of a three-dimensional coordinate system on the basis of a series of 2D ultrasound images which are obtained by scanning the subject sequentially along Z-axis of the coordinate system in a predetermined pitch. Normally, the resulting 3D ultrasound image is a non-transparent surface image simply showing boundary surfaces of a scanned range without exposing its internal structures to view. From non-transparent 3D ultrasound images of this sort, it is difficult to obtain information on the conditions of internal structures or internal tissues of a scanned subject. In order to expose internal structures to view, part of a 3D ultrasound image is hollowed out by an image cut-out operation at a position of particular interest to display on a monitor screen a cut-out 3D ultrasound image which permits to examine the conditions of internal structures or internal tissues of an organ three-dimensionally through a cut surface or surfaces of the displayed image.
For this purpose, firstly a plural number of 2D ultrasound images, i.e., unit images to be used for 3D image processing, are captured by the use of a known ultrasound imaging system having an ultrasound probe connected to an ultrasound image observation terminal with an ultrasound signal processor and a viewing screen. More specifically, by the use of such an ultrasound imaging system, a subject is scanned sequentially in a predetermined pitch by a B-mode ultrasound probe which is moved in a direction along the subject, to obtain an N-number of 2D ultrasound images as picture data for building up a 3D ultrasound image. For instance, while moving a radial scan ultrasound probe in a linear or axial direction thereof, a series of 2D ultrasound images are captured sequentially in a predetermined pitch or at predetermined intervals to provide unit images to be integrated into a 3D ultrasound image.
The unit images which have been captured in this manner by the use of a 2D ultrasound image processing system are fed to and processed by a 3D image processing system to produce a 3D ultrasound image. In the 3D image processing system, by coordinate conversion, picture data of the 2D ultrasound image are put on X- and Y-axes of a 3D coordinate system, which intersect with each other at an angle of 120.degree.. The coordinate-converted picture data of 2D ultrasound images are then aligned in the direction of Z-axis to build up a 3D ultrasound image of a columnar shape. The columnar 3D ultrasound image which is displayed on a monitor screen consists of three elemental images, i.e., images of two opposite end faces and a circumferential surface of a cylindrical scanned range. Namely, the 3D ultrasound image is of surfaces at outer boundaries of the scanned range, without showing internal structures which exist within the scanned range. Therefore, there arises a need for hollowing out part of the 3D ultrasound image, which is on display on a viewing screen, in a region of particular interest by an image cut-out operation. In the case of a 3D ultrasound image with such a cut-out portion, the elemental images include images of exterior surfaces of a scanned range as well as images of one or more cut surfaces. By changing the cut-out mode or cut position, the shape of an internal organ of particular interest or other internal structures or conditions of internal tissues can be viewed through cut surfaces of the cut-out portion.
Thus, conditions of an internal organ or tissues of particular concern can be observed through the cut surfaces which can be determined arbitrarily. In addition, by changing or shifting the positions of cut surface in 3D ultrasound images, one can grasp three-dimensional expansions of a subject more perfectly. Nevertheless, it is desirable for the 3D image processing system to be able to produce a cut-out 3D ultrasound image of this sort by simple signal processing operations, with quick response not only in signal processing for 3D ultrasound images but also in re-processing picture signals and refreshing a 3D image on display in response to alterations of cut-out mode or position. For this purpose, upon changing a cut-out mode or position, of a plural number of elemental picture images which constitute a 3D ultrasound image or a cut-out 3D ultrasound image, it suffices for the 3D signal processor to compile picture data of only those elemental images which correspond to new cut surfaces, and to paste the elemental images of new cut surfaces in specified positions on an original 3D ultrasound image. Accordingly, a 3D ultrasound image can be generated and refreshed by operations involving an extremely reduced amount of picture data. It follows that 3D ultrasound images can be generated and displayed in a desired cut-out mode quickly in a simplified manner.
Three-dimensional grasping of internal structures of a subject can be realized either by generating voxel data as explained hereinbefore in connection with the prior art by generating a cut-out 3D ultrasound image as in the above-mentioned patent by the present inventor. Whichever is the case, it becomes necessary to capture a large number of 2D ultrasound images sequentially by moving an ultrasound transducer in a direction perpendicular to ultrasound scan planes.
A 3D ultrasound image of high resolution can be obtained by processing picture data of all of captured 2D ultrasound images as unit images for a 3D image to be built up as described above. However, the greater the number of unit images, the larger becomes the amount of picture data to be processed. Accordingly, there may arise a necessity for selecting and extracting a suitable number of unit images for a 3D ultrasound image from numerous 2D ultrasound images captured. Above all, in the case of a 3D ultrasound image processing system as disclosed in the above-mentioned patent by the inventor, for the purpose of simplifying picture data processing for cut-out 3D ultrasound images, it is desirable for an image processing system to have a quick mode which depicts a relatively simple 3D image of internal structures by using only part of captured 2D ultrasound images as unit images, in addition to a fine mode using all of captured 2D ultrasound images to display a 3D ultrasound image of high resolution.
Further, when an ultrasound transducer is moved manually for capturing 2D unit images, there may arise a situation that, due to sluggish movements of the ultrasound transducer, same 2D ultrasound image are outputted from a 2D image capture device repeatedly before a displayed picture image is renewed. The same 2D picture images of this sort have to be removed prior to 3D image processing. Accordingly, in a 3D image processing on the basis of picture data of 2D ultrasound images which have been directly saved in an image recording means from a 2D image capture device, it becomes necessary to extract and distinguish unit images to be used for 3D ultrasound image processing, from other images in a large number of sequentially captured 2D images stored in an image recording means.
In selectively extracting unit images for 3D image processing from a large number of sequentially captured 2D ultrasound images as described above, field images of necessary unit images have to be instantly distinguishable from other field images. In this regard, since each 2D ultrasound image contains data of its own position (position data in the direction of Z-axis), arrangements may be made to extract the necessary unit images on the part of a 3D image processing system. However, it will lead to complication of the 3D processing system and of signal processing operations to be performed by the 3D processing system.